The fluctuation of human body temperature is one of the key indicators to assess health status, and although physiological standards are universally applicable, there are differences in the setting of specific standards from country to country, which has caused a lot of confusion and inconvenience to the public in practice.

Recently, a Chinese student who departed from Houston and transferred to Shanghai via Amsterdam, the Netherlands, was denied boarding a flight to China because his body temperature reached 4.0°C, slightly higher than the fever standard of 0.0°C set in China. The international student mentioned that when he boarded a plane in New York, he successfully passed the inspection despite his body temperature of 0.0°C. In fact, more than a dozen international students have already encountered similar difficulties because their body temperature is slightly higher than the domestic standard.

Why is there a difference in fever standards between China and the United States? What are the calculation and formulation principles behind this?

A person's body temperature is not constant, but fluctuates with different times of the day. Body temperature usually peaks in the evening and is relatively low in the early morning. Therefore, if the limit of fever is simply 3.0°C, many people may be considered to be in a state of fever in the evening. In addition, body temperature rises rapidly after physical activity.

In addition to the time factor, there are also differences in body temperature in different parts of the human body. According to clinical data, the anus is the site with the highest body temperature of 8.0°C, followed by the oral temperature of about 0.0°C, and the armpit temperature is relatively low at about 0.0°C.

The standard of body temperature and fever in China has been clearly divided: the first level is 40.0-0 °C, the second level is 0-0.0 °C, the third level is 0.0-0.0 °C, and the fourth level is 0.0-0 °C. Under this criterion, grade 1 and grade 2 are considered low-grade fevers, while grade 3 and above are considered high-grade fevers.

In contrast, the standards in the United States are different. Mild fever is defined as 5-0.0°C, moderate fever is 0.0-0.0°C, and severe fever is 0-0°C. Obviously, by American standards, 0.0°C is considered only moderate fever. In addition, the United States also considers the body temperature of different parts when determining whether there is a fever. During a medical examination, the temperature of the armpits alone is not enough to determine whether a fever is present, and the temperature of the mouth and anus is also measured. A fever is only considered a fever when the body temperature is elevated in multiple parts of the body.

Because of this significant difference, it is natural that different criteria will arise when taking body temperature. However, many people mistakenly believe that the United States has raised the fever standard to reduce the number of patients, and in fact this standard has always been the case.

中國的發燒標準則一直遵循著170年前的經典研究。1851年，由卡爾·溫德利希醫生創立的體溫標準，基於他對25000人、超過100萬個腋窩體溫數據的分析，得出人體的平均正常體溫是37度，波動範圍在36.2到37.5度之間。

However, in year 3, medical staff found that for most people, the armpit temperature is the lowest, so the fever threshold of 0.0°C does not apply to oral temperature. They suggest that when taking body temperature, differentiation should be made based on the temperature of different parts. However, in terms of standard setting and measurement, China has not yet made corresponding adjustments.

At the same time, the overall body temperature level of the human body is quietly changing. Researchers at Stanford University in the United States compared body temperature data from 6 years with records from the 0th century and found that the body temperature of modern men has dropped by 0.0°C compared to the past, while that of women has dropped by 0.0°C. From the average body temperature of 0 degrees in the 0th century, it has dropped to 0.0 degrees in modern times.

As for the cause of the drop in human body temperature, the medical community has not yet reached a definite conclusion. However, it is generally believed that this is closely related to changes in the social structure and the progress of science and technology. In the past, people had to exert a lot of physical effort to get food, but modern life has caused people's metabolism to generally slow down, and body temperature naturally drops. In addition, advances in medical technology have reduced the probability of getting sick and the average body temperature has been kept low. Interestingly, scientists have also found that a decrease in body temperature may help to prolong human lifespan, as the depletion and consumption of bodily functions are reduced in a hypothermic state.

Although the change in overall body temperature is a common trend for all humans from a macro perspective, for individuals, subtle differences in body temperature have a non-negligible impact on the state of the body. For example, due to a slowdown in metabolism, the body temperature of older people is lower than that of younger people. This drop in body temperature not only makes them more susceptible to cold when the temperature drops, but also their body coordination may be affected due to the constriction of blood vessels.

So, for each of us, while changes in overall body temperature may be less noticeable in the short term, ensuring that our body temperature matches our age is essential to maintaining a healthy state of health. Maintaining the right body temperature helps us to maintain our best physical condition in our daily lives.